On 8 January 2017, a strong earthquake with Mw 6.1 occurred 8° north of the Arctic Circle and 90 ... more On 8 January 2017, a strong earthquake with Mw 6.1 occurred 8° north of the Arctic Circle and 90 km southeast of Resolute, Nunavut, Canada. Because the epicenter was in a very remote region, the seismic station coverage was not good. As such, the errors in the source parameters determined using conventional procedures were not small. In this article, some results obtained on the source parameters by processing waveform records and modeling procedures are introduced. The teleseismic depth phase sP was used to determine the focal depths of the mainshock and the two principal aftershocks, because the nearest seismic station was at a distance of ∼90 km. The selected mantle Rayleigh-wave records surrounding the epicenter were used to invert for the moment tensor of the mainshock, and the nodal plane corresponding to the rupture plane was selected by analyzing the arrival-time differences between the Sg and Pg or Sn and Pn phases at the three close stations. Based on the focal depth values, it was found that the mainshock and its two principal aftershocks occurred within the lower crust. The fault plane was inferred to strike southeast and dip to the southwest, from the relocated hypocenters of the mainshock and the two principal aftershocks. A procedure using the calculated sP-P time duration to quickly determine focal depth was established. An average crustal model surrounding the epicenter was retrieved using Rayleigh-wave dispersion data.
ABSTRACT For small earthquakes, focal depths can be estimated jointly when epicenters are located... more ABSTRACT For small earthquakes, focal depths can be estimated jointly when epicenters are located using the arrival times of Pg and Sg waves recorded at seismic stations close to the event. However, if regional network coverage is sparse, this approach does not give accurate results. An alternative solution is the use of the regional depth-phase modeling (RDPM) method when such depth phases are available. Small, shallow earthquakes can generate Rg waves, the amplitudes of which approximately attenuate exponentially with focal depth; whereas, the amplitudes of Sg waves are, on average, less dependent on focal depth. Based on these features, a method using the maximum power spectral ratio (MPSR) between the Rg and Sg segments was developed to determine focal depth. Tests show the focal depth solutions obtained by the MPSR and RDPM methods for five events in an earthquake swarm and one event acquired by inspection are in good agreement. The error in the MPSR-determined focal depth caused by the error in the epicentral distance is in the order of 0.1km. The error in the focal depth when using a default focal mechanism is in the order of 0.5km. The quality factor, Q does not generate a significant error. Using the average of focal depths can provide a more reliable solution. Using an azimuth of approximately 45° from the strike direction to generate the synthetic ratio curve can reduce the error. As with any other earthquake locating technique, a reasonable regional crustal model is required when the MPSR method is used. Case studies show that the MPSR method can be used to successfully determine focal depths for events as small as m N 1.6. KeywordsDepth determination–Small shallow earthquakes– Rg/Sg spectral ratio
Earthquakes - Recent Advances, New Perspectives and Applications
On 23 June 2014, an MW 7.9 earthquake occurred in the Rat Islands region, Alaska, United States. ... more On 23 June 2014, an MW 7.9 earthquake occurred in the Rat Islands region, Alaska, United States. We inverted the full moment tensor for the mainshock, and found the shallow-dip nodal plane (P1) is: strike 207.4°, dip 27.1°, slip −12.7°; the steep-dip plane (P2) is: strike 308.7°, dip 84.2°, slip −116.5°. The larger aftershocks that have depth phase records were relocated and found the majority were distributed along a moderate dipping trend. The steep-dip plane was selected as the causative plane. Using the steep-dip plane as the rupture plane, source rupture process inversions were performed. The obtained maximum slip was about 3.5 m. The optimal rupture velocity VR was about 2.0 km/s. The shallow-dip plane was also used as a rupture plane to perform rupture inversion trials. Curiously the overall waveform fit between the observed and the synthetic seismograms is slightly better than that when the steep-dip plane was used. The catalogue hypocenters of the aftershocks with magnitude...
Earthquakes - Recent Advances, New Perspectives and Applications [Working Title]
On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with mb 5.7 oc... more On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with mb 5.7 occurred. It was followed by extensive aftershocks and felt throughout eastern Canada and north-eastern USA. Since this earthquake occurred in an uninhabited region, the damage was minor. Due to an mb 5.7 event is rare in north-eastern America, investigating it and its aftershocks is important for understanding intraplate seismicity. Digital seismic stations were not yet common by 1982. Fortunately, four seismic phases at three stations could be used to locate larger aftershocks. A simplified master-event location method combined with regional depth-phase modeling was used to locate aftershocks. For each aftershock its focal depth was first determined using a depth phase; then, with the depth fixed, the epicenter was determined using the four arrival time readings measured at the same three stations. The located aftershocks were divided into three groups. In each group the earthquake numbe...
On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with body-wave... more On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with body-wave magnitude (mb) 5.7 occurred. Itwas followed by extensive aftershocks and felt throughout eastern Canada and northeastern USA. Digital seismic stations were not yet commonby 1982. Fortunately, three stations (KLN, EBN and GGN) produced excellent waveform records for the larger aftershocksallowingthese aftershocks to be relocated. For each aftershock,its focal depth was first determinedusing thedepth phase sPg; then, with depth fixed, the epicenter was determinedusing a set of arrival times recorded for the Pg-, Sg-, and Pn-phases at the three stations.Sixty-eight aftershocks were relocated; mostof them occurred in a 5×5 km areaand with depthsof2 to 6 km. The epicentres formed two linear trends in theNE-SW direction. The trends were close to the northeast strike of the focal mechanism of the mainshock and consistent with the topographic trend near the source region. A gap betweenthe trends separated the epicenters into two groups. One group representstherupture area caused by themainshock, and the other groupmight represent the rupture area caused by the mb 5.4principal aftershock.
On 16 July 2014, two moderate earthquakes with similar body‐wave magnitude ( M B 4.5 and 4.6) an... more On 16 July 2014, two moderate earthquakes with similar body‐wave magnitude ( M B 4.5 and 4.6) and epicentral location occurred within 9 min in the northern Canadian Cordillera (NCC), northwestern Canada. Based on magnitude and waveform similarity, cataloged location, and timing, we refer to them as double earthquakes. In this study, we report their source properties using seismic data from regional and distant networks of broadband seismograph stations. Based on teleseismic depth‐phase modeling and double‐difference relocation, the focal depth for the first earthquake is ∼3.5–4.0 km and for the second, ∼6.5–7.0 km. The distance between the two epicenters is ∼1.6 km, for a total separation of less than 3 km. Moment tensor solutions show reverse slip with a minor strike‐slip component and a moment magnitude M w 4.2 for the first earthquake; the solution for the second earthquake is mainly strike slip with a small reverse‐slip component and a moment magnitude of M w 4.4. We analy...
Models of the seismic velocity structure of the crust in the seismically active northern Canadian... more Models of the seismic velocity structure of the crust in the seismically active northern Canadian Cordillera remain poorly constrained, despite their importance in the accurate location and characterization of regional earthquakes. On 29 August 2014, a moderate earthquake with magnitude 5.0, which generated high-quality Rayleigh wave data, occurred in the Northwest Territories, Canada, ∼100 km to the east of the Cordilleran Deformation Front. We carefully selected 23 seismic stations that recorded the Rayleigh waves and divided them into 13 groups according to the azimuth angle between the earthquake and the stations; these groups mostly sample the Cordillera. In each group, we measured Rayleigh wave group velocity dispersion, which we inverted for one-dimensional shear-wave velocity models of the crust. We thus obtained 13 models that consistently show low seismic velocities with respect to reference models, with a slow upper and lower crust surrounding a relatively fast mid crusta...
The 28 October 2012 Haida Gwaii, British Columbia, Canada, earthquake with a moment magnitude (MW... more The 28 October 2012 Haida Gwaii, British Columbia, Canada, earthquake with a moment magnitude (MW) of 7.8 occurred along an east-dipping poorly known thrust fault beneath the Queen Charlotte Terrace. It was the largest thrust event ever recorded in this dominated by strike-slip motion region. We studied the focal mechanism and the source rupture process for the event. The retrieved geometric parameters of the fault plane were a strike of 329°, dip of 24°, and slip of 114°. The isotropic moment was negative, and its value was about one-fifth of the total seismic moment released. The earthquake ruptured an area of about 160 km × 60 km, and major slip occurred in an area of about 100 km × 60 km. The maximum slip was about 5.8 m. The slip distribution on the fault plane was highly heterogeneous, with four slip patches. The main slip lay on a large zone above the hypocentre to the sea floor. The maximum and average stress drops calculated using the Brune model were 16.5 and 4.6 MPa, resp...
Since the Saguenay earthquake of 1988 four moderate magnitude earthquakes - 19901019 Mont-Laurier... more Since the Saguenay earthquake of 1988 four moderate magnitude earthquakes - 19901019 Mont-Laurier (Mw 4.5, depth=13 km), 19971106 Cap-Rouge (Mw 4.5, z=22), 19990316 Cote-Nord (Mw 4.5, z=19) and 20000101 Kipawa (Mw 4.7, z=12) - have occurred in southeastern Canada (together with three adjacent US events of similar size: 1998 Pymatuning, 2002 Au Sable Forks and 2002 Evansville). Improvements to seismograph coverage (particularily installation of digital broadband 3-cpt sensors) have provided higher quality data and allowed more detailed modelling of their rupture properties. Aftershock studies and seismotectonic investigations have provided new insights (we thank Maurice Lamontage for his contribution to these studies). We sumarize the published conclusions from the four earthquakes together with new surface-wave deduced parameters. These were typically thrust faulting events in response to NE-directed compression, and can be related to broad seismotectonic features in their vicinity,...
Bulletin of the Seismological Society of America, 2014
The source parameters of the 23 August 2011 M w 5.7 Virginia earthquake have been further studied... more The source parameters of the 23 August 2011 M w 5.7 Virginia earthquake have been further studied. To perform our source parameter review study, we first retrieved waveform records from Incorporated Research Institutions for Seismology. We then carefully selected 17 stations that had good Rayleigh-wave records. We used these 17 vertical records to retrieve a crustal shear-wave velocity model surrounding the epicenter. Using the regional depth phase modeling method, we retrieved focal depths for the mainshock and seven larger aftershocks, which ranged from about 3 to 10 km. With the retrieved focal depths, the arrival times of the P and S phases at four close stations and a revised hypoDD program package, we relocated the mainshock and seven larger aftershocks. On the vertical profile with an azimuth of 119°, the hypocenter trend was at 52°down from horizontal. The moment tensor of the mainshock was inverted by modeling long-period Rayleigh-wave records at 11 stations selected with consideration of a balanced distribution around the epicenter. Combined with the hypocenter trend, a plane solution with a strike of 29°, a dip of 55°, and a slip of 113°was identified to be close to the real rupture plane of the mainshock. The rupture process of the mainshock was preliminarily analyzed using regional waveform records. At least four subevent ruptures were found. The initiate depth of the first subevent rupture was at about 7.8 km, and the rupture speed was at about 1:3 km=s. The time gap between subevent ruptures 1 and 2 was about 0.34 s.
The earthquake was felt to distances exceeding 500 km. The "Felt Earthquake" form on the Geologic... more The earthquake was felt to distances exceeding 500 km. The "Felt Earthquake" form on the Geological Survey of Canada's Web pages received more than 450 submissions from more than 170 communities regarding this earthquake. The shaking was very strong for residents within 50 km of the epicenter (maximum intensity MM VI), and minor damage (fallen light objects, one broken ventilation pipe, and cracks in plaster) was reported. The seismograph station EEO (Eldee, Ontario), at a distance of 29 km, recorded a 0.023 g peak vertical acceleration, and a digital strong-motion unit operated by Hydro-Quebec at 68 km distance recorded 0.014 g maximum horizontal acceleration. Twenty-three aftershocks were recorded, ranging from M L 1.1 to raN2.2. To supplement station EEO, two portable recorders and a digital strong-motion instrument were installed on 2 January 2000 at distances of 9 to 22 km from the epicenter of the main shock. Preliminary analysis of the main shock from P first motions and surface-wave modeling indicates thrust faulting on an east-west or northwest-southeast trending plane. A depth of 13-16 km is indicated by the regional and teleseismic body waves. The epicenter is within 15 km of that of the magnitude (Mvr 6.1, 1935 Timiskaming earthquake and lies in a cluster of 76 earthquakes located since 1935. The area averages one magnitude 3 or greater earthquake every two years. There is an indication of a northwest-southeast elongation (parallel to the major faults of the Lake Timiskaming Graben), and focal mechanisms consistently show a northeast-dipping northwest-trending plane, which would be consistent with a fault outcropping under the Ottawa River and Lake Timiskaming.
ABSTRACT The basic parameters for the earthquake with a moment magnitude (M W) of 5.2 on the 23rd... more ABSTRACT The basic parameters for the earthquake with a moment magnitude (M W) of 5.2 on the 23rd of June 2010 have been investigated. The earthquake occurred on a hidden fault in the northwest direction about 60 km north-northeast of Ottawa in the Western Quebec Seismic Zone (WQSZ) and had a focal depth of about 21 km. The focal mechanism was a thrust type with strike in the northwest direction and dipping in the northeast direction. The relative relocations of seven larger aftershocks show that the source rupture area was about 6 km2. The b value of the aftershock sequence was 0.8–1.0, and the decay rate of the aftershocks was faster than normal cases. The dominant seismogenic depths are about 12 to 22 km in most parts of the WQSZ, while the seismogenic depth along the Ottawa–Bonnechere Graben can be as deep as 28 km. Based on the seismic activity in the WQSZ and vicinity since 1961, it seems that the periods of moderate earthquakes are about 6–10 years.
ABSTRACT Many crucial tasks in seismology, such as locating seismic events and estimating focal m... more ABSTRACT Many crucial tasks in seismology, such as locating seismic events and estimating focal mechanisms, need crustal velocity models. The velocity models of shallow structures are particularly important in the simulation of ground motions. In southern Ontario, Canada, many small shallow earthquakes occur, generating high-frequency Rayleigh (Rg) waves that are sensitive to shallow structures. In this research, the dispersion of Rg waves was used to obtain shear-wave velocities in the top few kilometers of the crust in the Georgian Bay, Sudbury, and Thunder Bay areas of southern Ontario. Several shallow velocity models were obtained based on the dispersion of recorded Rg waves. The Rg waves generated by an m N 3.0 natural earthquake on the northern shore of Georgian Bay were used to obtain velocity models for the area of an earthquake swarm in 2007. The Rg waves generated by a mining induced event in the Sudbury area in 2005 were used to retrieve velocity models between Georgian Bay and the Ottawa River. The Rg waves generated by the largest event in a natural earthquake swarm near Thunder Bay in 2008 were used to obtain a velocity model in that swarm area. The basic feature of all the investigated models is that there is a top low-velocity layer with a thickness of about 0.5 km. The seismic velocities changed mainly within the top 2 km, where small earthquakes often occur.
The western Quebec seismic zone (WQSZ) is a 160-km-wide band of intraplate seismicity extending 5... more The western Quebec seismic zone (WQSZ) is a 160-km-wide band of intraplate seismicity extending 500 km from the Adirondack Highlands (United States) to the Laurentian uplands (Canada). Previous authors have proposed that the WQSZ is localized over the Mesozoic track of the Great Meteor hot spot. Here we explore this hypothesis further by investigating regional seismicity characteristics. Focal mechanisms for WQSZ earthquakes, including a new mechanism for a moderate (mN 4.5) earthquake, reveal a pattern of reverse-sense faulting with SW trending P axes changing to E-W in the southern part of the zone. We introduce a simple box-counting method to delineate spatial clusters, based on exceedance of random seismicity density. Combining this approach with focal depths from regional depth phase analysis, we find that seismicity with shallow focus (0-7 km) is characterized by a random spatial distribution, whereas earthquakes with an intermediate focal depth (8-18 km) are strongly clustered along a diffuse linear band trending N50°W. Earthquakes deeper than 18 km are confined to a few distinct clusters. These clusters are characterized by differing b values and, for at least one cluster, repeating events. Projection of hypocenters onto a deep seismic profile and comparison with preexisting crustal structures suggest that local reactivation of Precambrian structural features may have occurred; however, the Great Meteor hot spot track remains the only compelling explanation for the overall distribution of earthquakes. Proximity of seismicity clusters to historic and prehistoric earthquakes lends support to the hypothesis that modern seismicity may represent exceptionally long-lived aftershocks of large past events.
Precise and accurate earthquake hypocentres are critical for various fields, such as the study of... more Precise and accurate earthquake hypocentres are critical for various fields, such as the study of tectonic process and seismic-hazard assessment. Double-difference relocation methods are widely used and can dramatically improve the precision of event relative locations. In areas of sparse seismic network coverage, however, a significant trade-off exists between focal depth, epicentral location and the origin time. Regional depth-phase modelling (RDPM) is suitable for sparse networks and can provide focal-depth information that is relatively insensitive to uncertainties in epicentral location and independent of errors in the origin time. Here, we propose a hybrid method in which focal depth is determined using RDPM and then treated as a fixed parameter in subsequent double-difference calculations, thus reducing the size of the system of equations and increasing the precision of the hypocentral solutions. Based on examples using small earthquakes from eastern Canada and southwestern USA, we show that the application of this technique yields solutions that appear to be more robust and accurate than those obtained by standard double-difference relocation method alone.
Bulletin of the Seismological Society of America, 2007
On 20 October 2005 at 21:16 UTC, a moderate earthquake (m N 4.3) occurred in an area of low seism... more On 20 October 2005 at 21:16 UTC, a moderate earthquake (m N 4.3) occurred in an area of low seismicity within Georgian Bay, approximately 12 km north of Thornbury, Ontario (44.67Њ N, 80.46Њ W). Despite its moderate magnitude, it was exceptionally well recorded and is of particular interest because of its location 90 km from a proposed long-term storage facility for low-and medium-level nuclear waste. No damage was reported, but ground shaking was felt to a distance of 100 km. Within 24 hours after the mainshock, four portable seismograph systems were installed in the epicentral region. In total, eight events were recorded over a 4-day period, including a foreshock and six aftershocks. The unusually rich dataset from this moderate earthquake sequence enabled robust determination of hypocentral parameters, including well-constrained focal depths for most events. For the mainshock, we estimated a seismic moment of M 0 4.5 ן 10 14 N m and corner frequency of 3.7 Hz, based on a spectral fit using Brune's source model. Least-squares waveform inversion of P and S phases yielded a double-couple focal mechanism with a reversesense of slip and northwest-striking nodal planes. The reverse mechanism and midcrustal focal depths (10-12 km) are characteristic, in general, of more abundant seismicity located ϳ200 km northeast of this event in the western Quebec seismic zone. These parameters differ, however, from shallow (2-6 km) earthquakes, with predominantly strike-slip mechanisms, observed near Lake Erie ϳ200 km to the south. We attribute this north-south change in rupture mechanism to variations in crustal stress induced by postglacial isostatic rebound. Aeromagnetic data in and around the epicentral region reveal prominent northwest-striking lineations caused by Precambrian mafic dykes. Under midcrustal conditions, the dyke material is mechanically stronger than generally more felsic upper-crustal host rocks. We propose that where large dykes are favorably oriented with respect to the stress field, they may strongly influence the locations of intraplate earthquake rupture in Shield regions.
Earthquake focal depth is a critical parameter, and one that the Polaris deployment in southern O... more Earthquake focal depth is a critical parameter, and one that the Polaris deployment in southern Ontario set out to determine. However, for most earthquakes in southeastern Canada and its vicinity, even the Polaris seismograph spacing is too sparse for reliable focal depth estimation by traditional methods. Teleseismic depth phases are also too weak for most of the earthquakes. However, the
Many small earthquakes occur annually in Eastern Canada, but moderate to strong earthquakes are i... more Many small earthquakes occur annually in Eastern Canada, but moderate to strong earthquakes are infrequent. The 25 November 1988 MW 5.9 Saguenay mainshock remains the largest earthquake in the last 80 years in eastern North America. In this article, some aspects of that earthquake sequence were re-analyzed using several modern methods. The regional depth-phase modeling procedure was used to refine the focal depths for the foreshock, the aftershocks, and other MN ≥ 2.5 regional earthquakes. The hypocenters of 10 earthquakes were relocated using hypoDD. The spatial distribution of eight relocated hypocenters defines the rupture plane of the mainshock. The moment tensor for the mainshock was retrieved using three-component long-period surface wave records at station HRV (Harvard seismograph station) with additional constraints from P-wave polarities. One nodal plane is conclusively identified to be close to the rupture plane, and its strike is similar to the trend of the south wall of ...
On 8 January 2017, a strong earthquake with Mw 6.1 occurred 8° north of the Arctic Circle and 90 ... more On 8 January 2017, a strong earthquake with Mw 6.1 occurred 8° north of the Arctic Circle and 90 km southeast of Resolute, Nunavut, Canada. Because the epicenter was in a very remote region, the seismic station coverage was not good. As such, the errors in the source parameters determined using conventional procedures were not small. In this article, some results obtained on the source parameters by processing waveform records and modeling procedures are introduced. The teleseismic depth phase sP was used to determine the focal depths of the mainshock and the two principal aftershocks, because the nearest seismic station was at a distance of ∼90 km. The selected mantle Rayleigh-wave records surrounding the epicenter were used to invert for the moment tensor of the mainshock, and the nodal plane corresponding to the rupture plane was selected by analyzing the arrival-time differences between the Sg and Pg or Sn and Pn phases at the three close stations. Based on the focal depth values, it was found that the mainshock and its two principal aftershocks occurred within the lower crust. The fault plane was inferred to strike southeast and dip to the southwest, from the relocated hypocenters of the mainshock and the two principal aftershocks. A procedure using the calculated sP-P time duration to quickly determine focal depth was established. An average crustal model surrounding the epicenter was retrieved using Rayleigh-wave dispersion data.
ABSTRACT For small earthquakes, focal depths can be estimated jointly when epicenters are located... more ABSTRACT For small earthquakes, focal depths can be estimated jointly when epicenters are located using the arrival times of Pg and Sg waves recorded at seismic stations close to the event. However, if regional network coverage is sparse, this approach does not give accurate results. An alternative solution is the use of the regional depth-phase modeling (RDPM) method when such depth phases are available. Small, shallow earthquakes can generate Rg waves, the amplitudes of which approximately attenuate exponentially with focal depth; whereas, the amplitudes of Sg waves are, on average, less dependent on focal depth. Based on these features, a method using the maximum power spectral ratio (MPSR) between the Rg and Sg segments was developed to determine focal depth. Tests show the focal depth solutions obtained by the MPSR and RDPM methods for five events in an earthquake swarm and one event acquired by inspection are in good agreement. The error in the MPSR-determined focal depth caused by the error in the epicentral distance is in the order of 0.1km. The error in the focal depth when using a default focal mechanism is in the order of 0.5km. The quality factor, Q does not generate a significant error. Using the average of focal depths can provide a more reliable solution. Using an azimuth of approximately 45° from the strike direction to generate the synthetic ratio curve can reduce the error. As with any other earthquake locating technique, a reasonable regional crustal model is required when the MPSR method is used. Case studies show that the MPSR method can be used to successfully determine focal depths for events as small as m N 1.6. KeywordsDepth determination–Small shallow earthquakes– Rg/Sg spectral ratio
Earthquakes - Recent Advances, New Perspectives and Applications
On 23 June 2014, an MW 7.9 earthquake occurred in the Rat Islands region, Alaska, United States. ... more On 23 June 2014, an MW 7.9 earthquake occurred in the Rat Islands region, Alaska, United States. We inverted the full moment tensor for the mainshock, and found the shallow-dip nodal plane (P1) is: strike 207.4°, dip 27.1°, slip −12.7°; the steep-dip plane (P2) is: strike 308.7°, dip 84.2°, slip −116.5°. The larger aftershocks that have depth phase records were relocated and found the majority were distributed along a moderate dipping trend. The steep-dip plane was selected as the causative plane. Using the steep-dip plane as the rupture plane, source rupture process inversions were performed. The obtained maximum slip was about 3.5 m. The optimal rupture velocity VR was about 2.0 km/s. The shallow-dip plane was also used as a rupture plane to perform rupture inversion trials. Curiously the overall waveform fit between the observed and the synthetic seismograms is slightly better than that when the steep-dip plane was used. The catalogue hypocenters of the aftershocks with magnitude...
Earthquakes - Recent Advances, New Perspectives and Applications [Working Title]
On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with mb 5.7 oc... more On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with mb 5.7 occurred. It was followed by extensive aftershocks and felt throughout eastern Canada and north-eastern USA. Since this earthquake occurred in an uninhabited region, the damage was minor. Due to an mb 5.7 event is rare in north-eastern America, investigating it and its aftershocks is important for understanding intraplate seismicity. Digital seismic stations were not yet common by 1982. Fortunately, four seismic phases at three stations could be used to locate larger aftershocks. A simplified master-event location method combined with regional depth-phase modeling was used to locate aftershocks. For each aftershock its focal depth was first determined using a depth phase; then, with the depth fixed, the epicenter was determined using the four arrival time readings measured at the same three stations. The located aftershocks were divided into three groups. In each group the earthquake numbe...
On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with body-wave... more On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with body-wave magnitude (mb) 5.7 occurred. Itwas followed by extensive aftershocks and felt throughout eastern Canada and northeastern USA. Digital seismic stations were not yet commonby 1982. Fortunately, three stations (KLN, EBN and GGN) produced excellent waveform records for the larger aftershocksallowingthese aftershocks to be relocated. For each aftershock,its focal depth was first determinedusing thedepth phase sPg; then, with depth fixed, the epicenter was determinedusing a set of arrival times recorded for the Pg-, Sg-, and Pn-phases at the three stations.Sixty-eight aftershocks were relocated; mostof them occurred in a 5×5 km areaand with depthsof2 to 6 km. The epicentres formed two linear trends in theNE-SW direction. The trends were close to the northeast strike of the focal mechanism of the mainshock and consistent with the topographic trend near the source region. A gap betweenthe trends separated the epicenters into two groups. One group representstherupture area caused by themainshock, and the other groupmight represent the rupture area caused by the mb 5.4principal aftershock.
On 16 July 2014, two moderate earthquakes with similar body‐wave magnitude ( M B 4.5 and 4.6) an... more On 16 July 2014, two moderate earthquakes with similar body‐wave magnitude ( M B 4.5 and 4.6) and epicentral location occurred within 9 min in the northern Canadian Cordillera (NCC), northwestern Canada. Based on magnitude and waveform similarity, cataloged location, and timing, we refer to them as double earthquakes. In this study, we report their source properties using seismic data from regional and distant networks of broadband seismograph stations. Based on teleseismic depth‐phase modeling and double‐difference relocation, the focal depth for the first earthquake is ∼3.5–4.0 km and for the second, ∼6.5–7.0 km. The distance between the two epicenters is ∼1.6 km, for a total separation of less than 3 km. Moment tensor solutions show reverse slip with a minor strike‐slip component and a moment magnitude M w 4.2 for the first earthquake; the solution for the second earthquake is mainly strike slip with a small reverse‐slip component and a moment magnitude of M w 4.4. We analy...
Models of the seismic velocity structure of the crust in the seismically active northern Canadian... more Models of the seismic velocity structure of the crust in the seismically active northern Canadian Cordillera remain poorly constrained, despite their importance in the accurate location and characterization of regional earthquakes. On 29 August 2014, a moderate earthquake with magnitude 5.0, which generated high-quality Rayleigh wave data, occurred in the Northwest Territories, Canada, ∼100 km to the east of the Cordilleran Deformation Front. We carefully selected 23 seismic stations that recorded the Rayleigh waves and divided them into 13 groups according to the azimuth angle between the earthquake and the stations; these groups mostly sample the Cordillera. In each group, we measured Rayleigh wave group velocity dispersion, which we inverted for one-dimensional shear-wave velocity models of the crust. We thus obtained 13 models that consistently show low seismic velocities with respect to reference models, with a slow upper and lower crust surrounding a relatively fast mid crusta...
The 28 October 2012 Haida Gwaii, British Columbia, Canada, earthquake with a moment magnitude (MW... more The 28 October 2012 Haida Gwaii, British Columbia, Canada, earthquake with a moment magnitude (MW) of 7.8 occurred along an east-dipping poorly known thrust fault beneath the Queen Charlotte Terrace. It was the largest thrust event ever recorded in this dominated by strike-slip motion region. We studied the focal mechanism and the source rupture process for the event. The retrieved geometric parameters of the fault plane were a strike of 329°, dip of 24°, and slip of 114°. The isotropic moment was negative, and its value was about one-fifth of the total seismic moment released. The earthquake ruptured an area of about 160 km × 60 km, and major slip occurred in an area of about 100 km × 60 km. The maximum slip was about 5.8 m. The slip distribution on the fault plane was highly heterogeneous, with four slip patches. The main slip lay on a large zone above the hypocentre to the sea floor. The maximum and average stress drops calculated using the Brune model were 16.5 and 4.6 MPa, resp...
Since the Saguenay earthquake of 1988 four moderate magnitude earthquakes - 19901019 Mont-Laurier... more Since the Saguenay earthquake of 1988 four moderate magnitude earthquakes - 19901019 Mont-Laurier (Mw 4.5, depth=13 km), 19971106 Cap-Rouge (Mw 4.5, z=22), 19990316 Cote-Nord (Mw 4.5, z=19) and 20000101 Kipawa (Mw 4.7, z=12) - have occurred in southeastern Canada (together with three adjacent US events of similar size: 1998 Pymatuning, 2002 Au Sable Forks and 2002 Evansville). Improvements to seismograph coverage (particularily installation of digital broadband 3-cpt sensors) have provided higher quality data and allowed more detailed modelling of their rupture properties. Aftershock studies and seismotectonic investigations have provided new insights (we thank Maurice Lamontage for his contribution to these studies). We sumarize the published conclusions from the four earthquakes together with new surface-wave deduced parameters. These were typically thrust faulting events in response to NE-directed compression, and can be related to broad seismotectonic features in their vicinity,...
Bulletin of the Seismological Society of America, 2014
The source parameters of the 23 August 2011 M w 5.7 Virginia earthquake have been further studied... more The source parameters of the 23 August 2011 M w 5.7 Virginia earthquake have been further studied. To perform our source parameter review study, we first retrieved waveform records from Incorporated Research Institutions for Seismology. We then carefully selected 17 stations that had good Rayleigh-wave records. We used these 17 vertical records to retrieve a crustal shear-wave velocity model surrounding the epicenter. Using the regional depth phase modeling method, we retrieved focal depths for the mainshock and seven larger aftershocks, which ranged from about 3 to 10 km. With the retrieved focal depths, the arrival times of the P and S phases at four close stations and a revised hypoDD program package, we relocated the mainshock and seven larger aftershocks. On the vertical profile with an azimuth of 119°, the hypocenter trend was at 52°down from horizontal. The moment tensor of the mainshock was inverted by modeling long-period Rayleigh-wave records at 11 stations selected with consideration of a balanced distribution around the epicenter. Combined with the hypocenter trend, a plane solution with a strike of 29°, a dip of 55°, and a slip of 113°was identified to be close to the real rupture plane of the mainshock. The rupture process of the mainshock was preliminarily analyzed using regional waveform records. At least four subevent ruptures were found. The initiate depth of the first subevent rupture was at about 7.8 km, and the rupture speed was at about 1:3 km=s. The time gap between subevent ruptures 1 and 2 was about 0.34 s.
The earthquake was felt to distances exceeding 500 km. The "Felt Earthquake" form on the Geologic... more The earthquake was felt to distances exceeding 500 km. The "Felt Earthquake" form on the Geological Survey of Canada's Web pages received more than 450 submissions from more than 170 communities regarding this earthquake. The shaking was very strong for residents within 50 km of the epicenter (maximum intensity MM VI), and minor damage (fallen light objects, one broken ventilation pipe, and cracks in plaster) was reported. The seismograph station EEO (Eldee, Ontario), at a distance of 29 km, recorded a 0.023 g peak vertical acceleration, and a digital strong-motion unit operated by Hydro-Quebec at 68 km distance recorded 0.014 g maximum horizontal acceleration. Twenty-three aftershocks were recorded, ranging from M L 1.1 to raN2.2. To supplement station EEO, two portable recorders and a digital strong-motion instrument were installed on 2 January 2000 at distances of 9 to 22 km from the epicenter of the main shock. Preliminary analysis of the main shock from P first motions and surface-wave modeling indicates thrust faulting on an east-west or northwest-southeast trending plane. A depth of 13-16 km is indicated by the regional and teleseismic body waves. The epicenter is within 15 km of that of the magnitude (Mvr 6.1, 1935 Timiskaming earthquake and lies in a cluster of 76 earthquakes located since 1935. The area averages one magnitude 3 or greater earthquake every two years. There is an indication of a northwest-southeast elongation (parallel to the major faults of the Lake Timiskaming Graben), and focal mechanisms consistently show a northeast-dipping northwest-trending plane, which would be consistent with a fault outcropping under the Ottawa River and Lake Timiskaming.
ABSTRACT The basic parameters for the earthquake with a moment magnitude (M W) of 5.2 on the 23rd... more ABSTRACT The basic parameters for the earthquake with a moment magnitude (M W) of 5.2 on the 23rd of June 2010 have been investigated. The earthquake occurred on a hidden fault in the northwest direction about 60 km north-northeast of Ottawa in the Western Quebec Seismic Zone (WQSZ) and had a focal depth of about 21 km. The focal mechanism was a thrust type with strike in the northwest direction and dipping in the northeast direction. The relative relocations of seven larger aftershocks show that the source rupture area was about 6 km2. The b value of the aftershock sequence was 0.8–1.0, and the decay rate of the aftershocks was faster than normal cases. The dominant seismogenic depths are about 12 to 22 km in most parts of the WQSZ, while the seismogenic depth along the Ottawa–Bonnechere Graben can be as deep as 28 km. Based on the seismic activity in the WQSZ and vicinity since 1961, it seems that the periods of moderate earthquakes are about 6–10 years.
ABSTRACT Many crucial tasks in seismology, such as locating seismic events and estimating focal m... more ABSTRACT Many crucial tasks in seismology, such as locating seismic events and estimating focal mechanisms, need crustal velocity models. The velocity models of shallow structures are particularly important in the simulation of ground motions. In southern Ontario, Canada, many small shallow earthquakes occur, generating high-frequency Rayleigh (Rg) waves that are sensitive to shallow structures. In this research, the dispersion of Rg waves was used to obtain shear-wave velocities in the top few kilometers of the crust in the Georgian Bay, Sudbury, and Thunder Bay areas of southern Ontario. Several shallow velocity models were obtained based on the dispersion of recorded Rg waves. The Rg waves generated by an m N 3.0 natural earthquake on the northern shore of Georgian Bay were used to obtain velocity models for the area of an earthquake swarm in 2007. The Rg waves generated by a mining induced event in the Sudbury area in 2005 were used to retrieve velocity models between Georgian Bay and the Ottawa River. The Rg waves generated by the largest event in a natural earthquake swarm near Thunder Bay in 2008 were used to obtain a velocity model in that swarm area. The basic feature of all the investigated models is that there is a top low-velocity layer with a thickness of about 0.5 km. The seismic velocities changed mainly within the top 2 km, where small earthquakes often occur.
The western Quebec seismic zone (WQSZ) is a 160-km-wide band of intraplate seismicity extending 5... more The western Quebec seismic zone (WQSZ) is a 160-km-wide band of intraplate seismicity extending 500 km from the Adirondack Highlands (United States) to the Laurentian uplands (Canada). Previous authors have proposed that the WQSZ is localized over the Mesozoic track of the Great Meteor hot spot. Here we explore this hypothesis further by investigating regional seismicity characteristics. Focal mechanisms for WQSZ earthquakes, including a new mechanism for a moderate (mN 4.5) earthquake, reveal a pattern of reverse-sense faulting with SW trending P axes changing to E-W in the southern part of the zone. We introduce a simple box-counting method to delineate spatial clusters, based on exceedance of random seismicity density. Combining this approach with focal depths from regional depth phase analysis, we find that seismicity with shallow focus (0-7 km) is characterized by a random spatial distribution, whereas earthquakes with an intermediate focal depth (8-18 km) are strongly clustered along a diffuse linear band trending N50°W. Earthquakes deeper than 18 km are confined to a few distinct clusters. These clusters are characterized by differing b values and, for at least one cluster, repeating events. Projection of hypocenters onto a deep seismic profile and comparison with preexisting crustal structures suggest that local reactivation of Precambrian structural features may have occurred; however, the Great Meteor hot spot track remains the only compelling explanation for the overall distribution of earthquakes. Proximity of seismicity clusters to historic and prehistoric earthquakes lends support to the hypothesis that modern seismicity may represent exceptionally long-lived aftershocks of large past events.
Precise and accurate earthquake hypocentres are critical for various fields, such as the study of... more Precise and accurate earthquake hypocentres are critical for various fields, such as the study of tectonic process and seismic-hazard assessment. Double-difference relocation methods are widely used and can dramatically improve the precision of event relative locations. In areas of sparse seismic network coverage, however, a significant trade-off exists between focal depth, epicentral location and the origin time. Regional depth-phase modelling (RDPM) is suitable for sparse networks and can provide focal-depth information that is relatively insensitive to uncertainties in epicentral location and independent of errors in the origin time. Here, we propose a hybrid method in which focal depth is determined using RDPM and then treated as a fixed parameter in subsequent double-difference calculations, thus reducing the size of the system of equations and increasing the precision of the hypocentral solutions. Based on examples using small earthquakes from eastern Canada and southwestern USA, we show that the application of this technique yields solutions that appear to be more robust and accurate than those obtained by standard double-difference relocation method alone.
Bulletin of the Seismological Society of America, 2007
On 20 October 2005 at 21:16 UTC, a moderate earthquake (m N 4.3) occurred in an area of low seism... more On 20 October 2005 at 21:16 UTC, a moderate earthquake (m N 4.3) occurred in an area of low seismicity within Georgian Bay, approximately 12 km north of Thornbury, Ontario (44.67Њ N, 80.46Њ W). Despite its moderate magnitude, it was exceptionally well recorded and is of particular interest because of its location 90 km from a proposed long-term storage facility for low-and medium-level nuclear waste. No damage was reported, but ground shaking was felt to a distance of 100 km. Within 24 hours after the mainshock, four portable seismograph systems were installed in the epicentral region. In total, eight events were recorded over a 4-day period, including a foreshock and six aftershocks. The unusually rich dataset from this moderate earthquake sequence enabled robust determination of hypocentral parameters, including well-constrained focal depths for most events. For the mainshock, we estimated a seismic moment of M 0 4.5 ן 10 14 N m and corner frequency of 3.7 Hz, based on a spectral fit using Brune's source model. Least-squares waveform inversion of P and S phases yielded a double-couple focal mechanism with a reversesense of slip and northwest-striking nodal planes. The reverse mechanism and midcrustal focal depths (10-12 km) are characteristic, in general, of more abundant seismicity located ϳ200 km northeast of this event in the western Quebec seismic zone. These parameters differ, however, from shallow (2-6 km) earthquakes, with predominantly strike-slip mechanisms, observed near Lake Erie ϳ200 km to the south. We attribute this north-south change in rupture mechanism to variations in crustal stress induced by postglacial isostatic rebound. Aeromagnetic data in and around the epicentral region reveal prominent northwest-striking lineations caused by Precambrian mafic dykes. Under midcrustal conditions, the dyke material is mechanically stronger than generally more felsic upper-crustal host rocks. We propose that where large dykes are favorably oriented with respect to the stress field, they may strongly influence the locations of intraplate earthquake rupture in Shield regions.
Earthquake focal depth is a critical parameter, and one that the Polaris deployment in southern O... more Earthquake focal depth is a critical parameter, and one that the Polaris deployment in southern Ontario set out to determine. However, for most earthquakes in southeastern Canada and its vicinity, even the Polaris seismograph spacing is too sparse for reliable focal depth estimation by traditional methods. Teleseismic depth phases are also too weak for most of the earthquakes. However, the
Many small earthquakes occur annually in Eastern Canada, but moderate to strong earthquakes are i... more Many small earthquakes occur annually in Eastern Canada, but moderate to strong earthquakes are infrequent. The 25 November 1988 MW 5.9 Saguenay mainshock remains the largest earthquake in the last 80 years in eastern North America. In this article, some aspects of that earthquake sequence were re-analyzed using several modern methods. The regional depth-phase modeling procedure was used to refine the focal depths for the foreshock, the aftershocks, and other MN ≥ 2.5 regional earthquakes. The hypocenters of 10 earthquakes were relocated using hypoDD. The spatial distribution of eight relocated hypocenters defines the rupture plane of the mainshock. The moment tensor for the mainshock was retrieved using three-component long-period surface wave records at station HRV (Harvard seismograph station) with additional constraints from P-wave polarities. One nodal plane is conclusively identified to be close to the rupture plane, and its strike is similar to the trend of the south wall of ...
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